Enceladus is a tiny satellite, just 505 kilometres across, but unusually dense.

It also is the most reflective object in the solar system, radiating nearly all the sunlight that strikes its cold surface.

"The only thing similar on Earth is freshly fallen snow," says Dr Bonnie Buratti, a planetary astronomer at the NASAJet Propulsion Laboratory in Pasadena, California.

Much of the moon's surface is uncannily smooth and free of impact craters that dot older, geologically dead terrain, such as Enceladus' neighbouring moons, Dione and Mimas.

Mimas sports one massive scar from an impact so powerful it nearly broke the moon apart.

Scientists are looking for what process is behind Enceladus' ongoing facelifts. One theory is that volcanic eruptions are continually coating the moon's surface, though with temperatures of -184°C, the spewing or oozing material is not molten rock like lava, but some sort of ammonia or water ice, Buratti says.

Scientists have found examples of cryovolcanism, or ice volcanoes, on Neptune's moon Triton.

"The question is," says Buratti, "is there some kind of active volcanic process going on now."

Another possibility is that particles from one of Saturn's rings, the E ring, continually pummel the moon's surface, exposing fresh terrain.

Enceladus is believed to be the source of the hazy ring, though scientists do not yet know how material is escaping the moon's gravity field to coalesce into the ring.

Closest encounter

During this week's flyby, Cassini made its closest approach ever to Enceladus. But the spacecraft had to turn its cameras and spectrometers away from the moon for about an hour during the encounter to protect the delicate instruments from a possible debris strike as the spacecraft flew through the E ring.

The closest images were taken when Cassini was about 5700 kilometres from Enceladus. Other instruments, such as one to detect magnetic and electric fields, collected data throughout the encounter.

Cassini is expected to swing past Enceladus three more times this year. The probe's next target is Saturn's hazy orange-tinged moon Titan.

In January, Cassini released the Huygens probe onto Titan, where it snapped intriguing photos of the moon's surface while tasting its atmosphere with its instruments.

Cassini's next pass by Titan will be its fourth.

Cassini, which reached Saturn in June after a seven-year trip, is on a four-year mission to study Saturn, its enigmatic rings and 31 known moons.